Electron gun



y 1960 J. w. SCHWARTZ 2,935,642

ELECTRONGUN Filed July 22. 1957 2 Sheets-Sheet 1 izau moon IN V EN TOR. dAMEsVV. SCHWARTZ May 3, 1960 J. w. SCHWARTZ ELECTRON GUN 2 Sheets-Sheet 2 Filed July 22, 1957 0 65 4 wz wmo I INVENTOR. JAMES W. SCHWARTZ BY" Y grqevar ELECTRON GUN "JamesjW. Schwartz, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application July 2 2, 1957, Serial No. 673,234

" 7 Claims. (oi. as -15 This invention isdirected to an electron gun structure for use in acathode ray tube.

A cathode ray tube utilizes an electron gun for provid- 'ing a narrow beam of electrons which is scanned over a fluorescent screen within the tube. The electrons of the beam are derived from a heated, thermionic cathode electrode and are urged through an apertured control grid electrode by appropriate accelerating fields. The control grid electrode may be a conducting plate structure having -a single aperture or a plurality of apertures positioned close together and overlying a portion ofthe emitting surface ofthe cathode. There is a range of operating voltages applied to the control gridelctrode, thelowest yoltage of which-prevents electrons from passing through the control grid aperture. This-potential is known as the cut-offpdtential of the tube and is usually a potential negative to that of the cathode electrode. The other potentials' atwhich the controlgrid is operated are more pfositiveithan the cut-off potential and are applied to the control grid to vary. theelectron'flow through the control .gn'd aperture,

In normal tube operation, incoming signal pulses are applied between the cathode electrode and the control grid to supply a varying potential difference between these two electrodes to form a corresponding variation in .the

electron flow through the control 'grid aperture. This produces a modulated electron beam, which 'by appropri ateelectrode structure is focused on the fiuoreseentscree'h .ofthe tube. As the beam isdefiected across thesurface of the. fluorescent screen,.its modulation produces a vari- :a le light pattern, which is anoptical reproductioniof .the incoming signals applied to the tube, I :Cathode ray tubes of the type described, which are used for-television viewing purposes and utilizing a cathodecoritrol 'gridarrangement, may require an operative .po-ZI tentialrange-of control grid voltages of up. to .100'volts fromcutofl potential. A normal control voltagerange,

however, is in the order of 50 to '10 volts from. cut=oft to naqiimumbeam -current. These voltage ranges are excessive and require considerable amplification of the in}. coming video signal to operatethe tube effectively. I

-As the control grid is operated within its operative I range from cut-oft, the focus of-the electron beam at efluorescent screen varies with change in potential of fl thefcontrol grid. This is an undesired eiiectsin'ce'loss 'ofbe'am' focus at'the fluorescent's'creen reduces the resolution-ofthe picture. Another disadvantage of conventiona1 cathode ray 'tubesisthat the cathode emitting 'surtaceis aligned-withthe pre-defiection be'am path. Since the'high energy electron beam'will tend-to ionize' residual gas-molecules within the tube, .positive gas ions are formed and'are directed back-alongthe'be'am path and bombardthe exposed cathode emitting surface. Th s ttends to destroythe effectiveness of the cathode-andthus eho tensthe ife. o he h 1 i hus, a obi o t enti t P ovi e l-Q requirementsfor control grid operation) tron gun foracathodezray-tubewhiclrl asflo voltage -control a i stru t r c usis ing-xot iairof Pate t d M y 3., 1.969

ice

a It is another object of the inventiontoprovide a novel electron gun for a cathode ray tube requiring a: small change in control voltage from cut-01f] to full beam current. It [is a further object of the invention-to provide a novel electron-"gun for a cathode ray tube having low voltage requirements for the control grid electrode and in which the beam focus at the fluorescent screenis independent ofthebea'm current- I i The. invention is in a novel gun structureutilizing an annular cathode electrode having an emitting area on the inner surface of the annulus.

-face al t e of. the annular cathod t r ug a small object aperture in a beam masking electrode... Electrons P s ng r gh. theobiect aperture areifocused igure 4. is a graphical showing of the .operatingcharacteristics .of the novel gun structure shown in .Eigure 2 V under I certain specific conditions of operation.

Figures 5 and 6 are, partial sectional views showing modifications of the gun structure of Figure 2.:

Figure 1 shows a cathode ray tube utilized foritelevision purposes,*which consists of an evacuated; envelope having a bulb portion 10. and a tubular neckportionlz extending from the smaller-endof the:;.bulb 10 ,Within the envelopeneck portion 12 there is mountedanxelectron gun structure 14 consisting of a plurality .of electrodes aligned along the axis ofthe neckportion. The gun structure forms an electron beam from electrons emitted by a thermionic cathode electrode and directs the beam onto a fluorescent screen within the tube. The fluorescent screen consists of a thin layer 16 of phosphor material coated on the inner surface of a transparent'faceplate 18; closing the large :end of. the. tapered bulb 10. The electron beam is scanned over the surface of phosphor screen 16 by magnetic fields formed from appropriately designed-coils mountedwithin aneck Yoke structure 20.

The electron gun 14 is shown in detail inthe. enlarged.

formed as an annular ring structure having Fa Ll-shaped Qcross-section, as shown.

The cathode ring. 22 is fixed to. amount ring pos tioned on entr cally h he c hod i e 22-. Be e the t rings .2 and 2 an a mn mo d l i i ure 3 a e ap u ali y o met l t ps ln'e ch-hav ng on end fix d o'the cathode ring .2 a ross s =shaped channel port nandth ther end fixed tothe inner'sur-faceofthe support ring 24. upp rting structure permits he c thod rin 22 to retain its axiallposition'within the gun structurewhen the cathode ring 22 becomes heated to a high temperature dun' ing tube operation. An'vexp nsion of thecathode'fri 22 will cause the ring torotate about its axis slightlyand maintain its axial position'relative to the-other'g'un electrodes constant. Cathode mountingiing 24 is in turn Q supported by two metal studs. 28, eachiwelded at one end to. the ring 24 andhaving'the other endfixefcl'within'glass' support rods 30.

, *flh nner surfa e ofthe cath de'rina iscoated witli .o

311 th rmi nic mi t n material to provide-oc e e rons- 19 5! spa e f mhis apertured' circular control grid plates 32 andf-34 whose A control grid electrode and an accelerating electrode are formed as coaxial rings disposed within the annular cathode surface. A robe electrode directs electronsemitted by 'thehathodelsiir y peripheries are fixed into the glass support rods 30. Plate 32 has a short cylindrical extension or tubular member 36 projecting over the coated surface of cathode 22. In a similar manner, plate 34 has a short cylindrical projection or tubular member 38 extending axially toward the tubular member 36. The two plates 32 and 34 are in this example identical in all dimensions and are rigidly fixed to the glass rods so that they are coaxial with the annular cathode 22. The spacing between the first pair of tubular members 36 and 38 of the control grid plates forms an annular open space overlying the cathode surface and is utilized to control the electron emission from the adjacent surface of the cathode ring 22.

. -A second pair of circular plates 37 and 39 of identical size and shape, are supported by the glass rods 30. Plate 37 has a central aperture with a short cylindrical wall portion or tubular member 40 extending from the periphcry of the aperture and overlying the tubular member 36 of ring 32. Plate 39 in a similar manner, has a cylindrical wall extension or tubular member 42 extending from the periphery of its.central aperture toward the oppositely extending tubular member 40 of ring 37. The second pair of tubular members 40 and 42 are spaced apart to form an annular opening between them which is aligned with the annular opening between tubular members 36 and 38.

An electrode plate structure 44 is fixed to the glass rods 30 in a plane parallel to the plane of the circular plate 37 and closely spaced therefrom. At the center of plate 44, and symmetrically formed relative to the axis of the cathode ring 22, is a stud-like structure 46 having a somewhat concave conical surface of revolution 43 symmetrically disposed on the axis of the ring 22. The

stud structure 46 will be referred to as a probe electrode in the discussion of the operation of the gun structure below. 7

An object aperture electrode for selectively masking the electron beam is formed as a circular disk 50 having at A second accelerating eletrode 64 is coaxially aligned with the axis of the annular cathode 22. Electrode 64 is a tubular member having an enlarged portion 66 at the end farther from the cathode electrode. A conductive wall coating 68 is formed on the inner surface of the tube neck 12 and extends into the bulb portion 10 to a point adjacent to screen 16, as shown in Figure 1.

The cathode electrode 22 is heated by a heater filament 70 consisting of an electrical coil positioned within the channel of electrode 22. The control grid rings 32 and 34 are maintained at a common potential and are biased negatively with respect to the cathode electrode 22. Rings 37 and 39 are accelerating electrodes and are operated at a common positive potential or at difierent positive potentials as shown in Figure 2. The probe electrode 46 is held at ground potential or a few volts negative to cathode, while the apertured object electrode 50 is operated somewhere Within the range of 20 to 200 volts positive relative to ground potential, depending upon the desired operating characteristics of the tube. Accelerating electrode 56 is held within a range of 500 to 4,000 volts positive, while electrode 64 is operated at around 5,000 volts, and the wall coating electrode 68 at 20,000 volts.

'During tube operation, the control electrodes 32 and 34, the accelerating electrodes 37 and 39 and the probe electrode 46 produce an electrostatic field which first accelerates the electrons through the annular aperture between the control grid portions 36 and 38 and directs the accelerated electrons radially inward and then directs the accelerated electrons axially relative to the axis of cathode 22 and focuses or converges the electron fiow onto the apertured electrode 50, normal to the axis of cathode 22. The paths of the electrons will terminate at the object electrode 50 or will pass through the central aperture 54 in this electrode. "The electrons passing through the aperture 54 are accelerated and focused by the electron lens fields between electrodes 56 and 64 and between electrode portion 66 and coating 68 to cause the electron paths to converge to a small spot on the fluorescent screen 1 6. I

Figure 4 shows-characteristic curves derived from one operation of the tube. Variations in the control voltage applied to plates 32 and 34 are plotted against values of the electron current passing through aperture 54 of the gun. The curves are for different voltage conditions applied to accelerating electrodes 37 and 39, as indicated, in which E is the voltage applied to electrode 37 and B is the voltage applied to electrode 39. During these several operating conditions, the probe electrode 46 was operated at ground or cathode potential, the apertured electrode 50 at about 50 volts positive relative to ground, and accelerating electrode 56 at around 500 volts positive relative to ground.

Each of the curves of Figure 4 indicates a novel characteristic of the gun under the conditions of operation. The gun has cutoff of beam current at a negative potential greater than the 45 volts shown in Figure 4. The beam current through aperture 54 increases as the control grid (32, 34) becomes less negative and reaches a maximum at a relatively small negative potential relative to cathode potential. Then the current drops oif very quickly to a cut-off potential of the control grid which is still negative relative to cathode or zero potential. The slope of this current drop is very steep and the curves (to the right of the peaks) indicate that the tube may be operated with only a few volts drive between maximum current and cut-ofi potential which, as in the case of curve A, is as low as 5 volts. This small voltage drive is a characteristic of the novel gun structure and has a distinct advantage in that less amplification of the signal voltage is required before it is applied to the control grid electrodes.

The sudden drop in current as shown in Figure 4 from a maximum to a minimum at close to zero potential is due to a type of focus modulation of the electrons between the cathode surface and the aperture 54. When the tube is operated at the peak of the curve, the current passing through aperture 54 is a maximum, since the electron emission from the cathode surface is focused into aperture 54 to provide a high density of electrons within the aperture. However, as the control electrodes 32 and 34 are made more positive, the electron flow becomes defocused and there is a reduction in the electron density at the center of the electron stream which continues to drop until no current passes through aperture 54, and the beam current beyond aperture 54 falls to zero. These results ocur since the electrons coming from the cathode surface are focused to form a conical hollow beam in the region adjacent to accelerating electrode portion 52. The conical beam has an apex at the aperture 54 when the beam current is maximum, but as the control grid electrodes 32 and 34 are made more positive, the conical beam, because it becomes defocused, widens and more and more of it strikes the periphery of aperture 54 and the electrode portion surrounding the aperture until a point is reached where the recessed portion 52 surrounding aperture 54 collects all of the beam "electrons. Thus, the control grid electrodes 32-34 function to control the current flow from the cathode electrode 22, and also control the focus characteristics of the field formed by the electrodes between the cathode .a portion thereof coaxially mounted within said annular cathode, a plurality of annular electrodes including a control electrode, said electrodes having portions positioned between said reflector electrode and said cathode surface for providing accelerating and focus modulating electrostatic fields between said cathode and said reflector electrodes, and an accelerating electrode spaced from said cathode electrode along the axis thereof, said accelerating electrode having an aperture therethrough and on the axis of said annular cathode.

3. An electron gun for a cathode ray tube, said gun comprising an annular cathode electrode having an electron emitting inner surface, a reflector electrode having a portion thereof coaxially mounted within said annular cathode, and a plurality of annular electrodes including a control electrode, said electrodes having portions positioned between said reflector electrode and said cathode surface for providing accelerating and focus modulating electrostatic fields, between said cathode and said reflector electrodes, said annular electrode portions including a first pair of tubular members having substantially equal diameters mounted coaxially within said annular cath ode with adjacent ends spaced from and aligned with each other, and a second pair of tubular electrode members coaxially mounted within said cathode electrode between said first pair of tubular members and said reflector electrode, said second pair of tubular members being spaced along said cathode axis, said reflector electrode portion having a conical surface coaxial with said annular cathode, the spacings between said adjacent ends of said first pair of tubular members and between said second pair of spaced tubular electrodes being aligned with said cathode emitting surface and said conical reflector surface, and an accelerating electrode spaced along said cathode axis from said conical reflector surface, said accelerating electrode having a central aperture on said cathode axis.

4. An electron gun for a cathode ray tube, said gun comprising a cmthode electrode for providing a source of electrons, electrode means arranged adjacent to said cathode electrode for forming electrons from said cathode electrode into an electron flow, and an apertured electrode offset from said electron flow, said electrode means including structure between said cathode and said apertured electrode for focusing said electron flow onto said apertured electrode, said structure comprising means for 8 changing the focus of said electron flow on said apertured electrode and for controlling the electron current in said flow of electrons, said electrode means including a probe electrode positioned in the path of said electron flow to deflect said electron flow toward said aperture electrode.

5. An electron gun for a cathode ray tube, said gun comprising a cathode electrode having an arcuate concave electron emitting surface, a reflector electrode having a portion thereof concentrically mounted relative to said arcuate cathode surface, an apertured electrode spaced along the axis of said arcuate cathode surface from said reflector electrode, and a plurality of electrodes including a control electrode, said electrodes having portions positioned between said reflector electrode and said cathode surface for providing an accelerating electrostatic field between said cathode and said reflector electrode and for focusing electrons from said cathode surface onto said apertured electrode.

6. An electron gun comprising means for initiating an annular electron emission, means for establishing a flow of the electrons of said emission radially inward toward the axis of the annulus of said annular emission, means for deflecting said flow to along said axis, electric field means including means for accelerating said electrons radially inward and for selectively variably focusing said electron flow and electron beam masking means disposed in said axial beam path.

7. An electron gun comprising means for initiating an annular electron emission, means for establishing a flow of the electrons of said emission radially inward toward the axis of the annulus of said annular emission, means for creating an electric field to deflect said flow to along said axis, means for shaping said electron deflecting field to focus said electron flow along said axis, means for partially masking said electron flow at a point along said axis, and means for modulating the focusing effect of said field to vary the amount of electron flow past said point.

References Cited in the file of this patent UNITED STATES PATENTS 2,163,787 Henneberg et al. June 27, 1939 2,452,044 Fox Oct. 26, 1948 2,801,361 Pierce July 30, 1957 2,812,467 Kompfner Nov. 5, 1957 2,813,990 Robertson Nov. 19, 1957 

